1WebAssembly lld port 2==================== 3 4The WebAssembly version of lld takes WebAssembly binaries as inputs and produces 5a WebAssembly binary as its output. For the most part it tries to mimic the 6behaviour of traditional ELF linkers and specifically the ELF lld port. Where 7possible the command line flags and the semantics should be the same. 8 9 10Object file format 11------------------ 12 13The WebAssembly object file format used by LLVM and LLD is specified as part of 14the WebAssembly tool conventions on linking_. 15 16This is the object format that the llvm will produce when run with the 17``wasm32-unknown-unknown`` target. 18 19Usage 20----- 21 22The WebAssembly version of lld is installed as **wasm-ld**. It shared many 23common linker flags with **ld.lld** but also includes several 24WebAssembly-specific options: 25 26.. option:: --no-entry 27 28 Don't search for the entry point symbol (by default ``_start``). 29 30.. option:: --export-table 31 32 Export the function table to the environment. 33 34.. option:: --import-table 35 36 Import the function table from the environment. 37 38.. option:: --export-all 39 40 Export all symbols (normally combined with --no-gc-sections) 41 42 Note that this will not export linker-generated mutable globals unless 43 the resulting binaryen already includes the 'mutable-globals' features 44 since that would otherwise create and invalid binaryen. 45 46.. option:: --export-dynamic 47 48 When building an executable, export any non-hidden symbols. By default only 49 the entry point and any symbols marked as exports (either via the command line 50 or via the `export-name` source attribute) are exported. 51 52.. option:: --global-base=<value> 53 54 Address at which to place global data. 55 56.. option:: --no-merge-data-segments 57 58 Disable merging of data segments. 59 60.. option:: --stack-first 61 62 Place stack at start of linear memory rather than after data. 63 64.. option:: --compress-relocations 65 66 Relocation targets in the code section are 5-bytes wide in order to 67 potentially accommodate the largest LEB128 value. This option will cause the 68 linker to shrink the code section to remove any padding from the final 69 output. However because it affects code offset, this option is not 70 compatible with outputting debug information. 71 72.. option:: --allow-undefined 73 74 Allow undefined symbols in linked binary. This is the legacy 75 flag which corresponds to ``--unresolve-symbols=ignore`` + 76 ``--import-undefined``. 77 78.. option:: --unresolved-symbols=<method> 79 80 This is a more full featured version of ``--allow-undefined``. 81 The semanatics of the different methods are as follows: 82 83 report-all: 84 85 Report all unresolved symbols. This is the default. Normally the linker 86 will generate an error message for each reported unresolved symbol but the 87 option ``--warn-unresolved-symbols`` can change this to a warning. 88 89 ignore-all: 90 91 Resolve all undefined symbols to zero. For data and function addresses 92 this is trivial. For direct function calls, the linker will generate a 93 trapping stub function in place of the undefined function. 94 95.. option:: --import-memory 96 97 Import memory from the environment. 98 99.. option:: --import-undefined 100 101 Generate WebAssembly imports for undefined symbols, where possible. For 102 example, for function symbols this is always possible, but in general this 103 is not possible for undefined data symbols. Undefined data symbols will 104 still be reported as normal (in accordance with ``--unresolved-symbols``). 105 106.. option:: --initial-memory=<value> 107 108 Initial size of the linear memory. Default: static data size. 109 110.. option:: --max-memory=<value> 111 112 Maximum size of the linear memory. Default: unlimited. 113 114By default the function table is neither imported nor exported, but defined 115for internal use only. 116 117Behaviour 118--------- 119 120In general, where possible, the WebAssembly linker attempts to emulate the 121behaviour of a traditional ELF linker, and in particular the ELF port of lld. 122For more specific details on how this is achieved see the tool conventions on 123linking_. 124 125Function Signatures 126~~~~~~~~~~~~~~~~~~~ 127 128One way in which the WebAssembly linker differs from traditional native linkers 129is that function signature checking is strict in WebAssembly. It is a 130validation error for a module to contain a call site that doesn't agree with 131the target signature. Even though this is undefined behaviour in C/C++, it is not 132uncommon to find this in real-world C/C++ programs. For example, a call site in 133one compilation unit which calls a function defined in another compilation 134unit but with too many arguments. 135 136In order not to generate such invalid modules, lld has two modes of handling such 137mismatches: it can simply error-out or it can create stub functions that will 138trap at runtime (functions that contain only an ``unreachable`` instruction) 139and use these stub functions at the otherwise invalid call sites. 140 141The default behaviour is to generate these stub function and to produce 142a warning. The ``--fatal-warnings`` flag can be used to disable this behaviour 143and error out if mismatched are found. 144 145Exports 146~~~~~~~ 147 148When building a shared library any symbols marked as ``visibility=default`` will 149be exported. 150 151When building an executable, only the entry point (``_start``) and symbols with 152the ``WASM_SYMBOL_EXPORTED`` flag are exported by default. In LLVM the 153``WASM_SYMBOL_EXPORTED`` flag is set by the ``wasm-export-name`` attribute which 154in turn can be set using ``__attribute__((export_name))`` clang attribute. 155 156In addition, symbols can be exported via the linker command line using 157``--export`` (which will error if the symbol is not found) or 158``--export-if-defined`` (which will not). 159 160Finally, just like with native ELF linker the ``--export-dynamic`` flag can be 161used to export symbols in the executable which are marked as 162``visibility=default``. 163 164Imports 165~~~~~~~ 166 167By default no undefined symbols are allowed in the final binary. The flag 168``--allow-undefined`` results in a WebAssembly import being defined for each 169undefined symbol. It is then up to the runtime to provide such symbols. 170 171Alternatively symbols can be marked in the source code as with the 172``import_name`` and/or ``import_module`` clang attributes which signals that 173they are expected to be undefined at static link time. 174 175Garbage Collection 176~~~~~~~~~~~~~~~~~~ 177 178Since WebAssembly is designed with size in mind the linker defaults to 179``--gc-sections`` which means that all unused functions and data segments will 180be stripped from the binary. 181 182The symbols which are preserved by default are: 183 184- The entry point (by default ``_start``). 185- Any symbol which is to be exported. 186- Any symbol transitively referenced by the above. 187 188Weak Undefined Functions 189~~~~~~~~~~~~~~~~~~~~~~~~ 190 191On native platforms, calls to weak undefined functions end up as calls to the 192null function pointer. With WebAssembly, direct calls must reference a defined 193function (with the correct signature). In order to handle this case the linker 194will generate function a stub containing only the ``unreachable`` instruction 195and use this for any direct references to an undefined weak function. 196 197For example a runtime call to a weak undefined function ``foo`` will up trapping 198on ``unreachable`` inside and linker-generated function called 199``undefined:foo``. 200 201Missing features 202---------------- 203 204- Merging of data section similar to ``SHF_MERGE`` in the ELF world is not 205 supported. 206- No support for creating shared libraries. The spec for shared libraries in 207 WebAssembly is still in flux: 208 https://github.com/WebAssembly/tool-conventions/blob/master/DynamicLinking.md 209 210.. _linking: https://github.com/WebAssembly/tool-conventions/blob/master/Linking.md 211